This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods that employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply "carbonylation" for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy Group VIII metal; i.e., a Group VIII metal having an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof. Palladium catalysts have been found particularly useful; they include complexes with phosphines such as triphenylphosphine.
The production of carbonates may frequently be improved by including a metal-based cocatalyst along with the heavy Group VIII metal catalyst. Metal-based cocatalysts have been described broadly in U.S. Pat. No. 4,187,242, 4,201,721 and 5,380,907. Lead compounds as cocatalysts are particularly detailed in U.S. Pat. No. 5,498,789. Also preferred in general is the use of various halides, as illustrated by tetra-n-butylammonium bromide. Compounds characterized as inert solvents, such as toluene, diethyl ether, diphenyl ether and acetonitrile, can also be present.
Many of the catalyst systems known in the art have disadvantages such as low active catalyst lifetime, typically 2 hours or less, and low selectivity to the desired diaryl carbonate as a result of formation of relatively high proportions of by-products such as bromophenols.
Also, it has been observed that certain palladium-based analysts, such as palladium(II) acetate, show a decrease in catalytic activity upon storage in contact with hydroxyaromatic compounds such as phenol at temperatures on the order of 70.degree. C. for periods as short as 2 hours. This decrease is notable particularly when the palladium compound is present in catalyst mixtures containing lead(II) oxide.
It is of interest, therefore, to develop catalyst systems that have long lifetimes, not decreased by storage, and which improve selectivity.